In order to understand the object of the invention clearly, it is appropriate to recall the main types of flying machine corresponding to airplanes and to rotorcraft.
The term “rotorcraft” is used to designate any aircraft in which lift is provided in full or in part by one or more propellers of substantially vertical axis and of large diameter, referred to as rotors or as rotary wings.
The rotorcraft category is itself subdivided into several distinct types.
Firstly, there is the helicopter, having at least one main rotor that is driven by a suitable power plant and that provides both lift and propulsion.
Then there is the autogyro, which is a rotorcraft in which the rotor does not receive power, but provides lift by autorotating under the effect of the forward speed of the aircraft. Propulsion is provided by a turbine engine or by a propeller of axis that is substantially horizontal in forward flight and that is driven by a conventional engine.
The gyrodyne is a rotorcraft intermediate between the helicopter and the autogyro, in which the rotor provides lift only. The rotor is normally driven by a power plant during stages of takeoff, hovering or vertical flight, and landing, like a helicopter. A gyrodyne thus includes an additional propulsive system that is essentially different from the rotor assembly.
In forward flight, the rotor continues to provide lift, but solely in autorotation mode, i.e. without power being transmitted to said rotor.
Several other novel formulae have been studied to a greater or lesser extent, and some of them have given rise to practical embodiments.
In this context, mention may be made of the compound helicopter that takes off and lands like a helicopter and that flies like an autogyro when cruising: its rotor performs autorotation because of the forward speed of the aircraft and provides some of its lift, with the remainder of its lift being provided by an auxiliary wing. A tractor propeller of substantially horizontal axis delivers the force needed for movement in translation. By way of example, document GB 613 715 describes a compound helicopter.
Similarly, document U.S. Pat. No. 6,513,752 describes an aircraft comprising:                a fuselage and a wing;        two variable pitch propellers;        a rotor with heavy weights “at their ends” (sic);        a power plant driving the two propellers and the rotor;        control means for adjusting the pitch of the propellers such that:                    in forward flight, the thrust from the propellers is exerted towards the front of the aircraft; and            in hovering flight, the antitorque function is provided by one of the propellers delivering thrust towards the front and the other towards the rear of the aircraft, with the rotor being driven by the power plant; and                        the power plant comprises an engine and a clutch that, by disconnecting the rotor from the engine, enables the rotor to rotate faster than an outlet from the engine, because of the above-mentioned weights.        
In addition, it is specified that the clutch makes autogyro mode possible in forward flight. Consequently, the aircraft according to document U.S. Pat. No. 6,513,752 is of the compound type.
In addition, a power transmission gearbox disposed between the power plant and the propellers enables said propellers to operate at a plurality of different speeds of rotation relative to the speed at the outlet from said power source.
The convertible constitutes another particular formula for a rotorcraft. This term covers all rotorcraft that change configuration while in flight: takeoff and landing in a helicopter configuration; cruising flight in an airplane configuration; with two rotors being tilted through about 90 degrees as to act as propellers, for example.
Another novel formula is known that is referred to as a “hybrid” helicopter for convenience.
The hybrid helicopter has a fuselage with a main rotor for rotating blades under drive from at least one turbine engine.
In addition, the hybrid helicopter is provided with a wing made up of two half-wings, with two propulsive propellers being disposed on either side of the fuselage, on respective ones of the half-wings.
In addition, the hybrid helicopter has an integrated drive train that comprises not only the turbine engine(s), the rotor, and the two propellers, but also a mechanical system interconnecting those elements.
With this configuration, the speeds of rotation at the outlet(s) from the turbine engine(s), of the propellers, of the rotor, and of the mechanical interconnection system are all mutually proportional, with the proportionality ratio being constant regardless of the flying configuration of the hybrid helicopter under normal conditions of operation of the integrated drive train.
Consequently, and advantageously, the rotor is always driven in rotation by the turbine engine(s) and always delivers lift regardless of the configuration of the hybrid helicopter, both in forward flight and in hovering flight. The hybrid helicopter is thus neither an autogyro, nor a gyrodyne, nor a compound, but a novel type of rotorcraft.
More precisely, the rotor is designed to provide all of the hybrid helicopter's lift during stages of takeoff, landing, and vertical flight, and to provide some of its lift during cruising flight, with the wing then contributing a fraction of the lift of said hybrid helicopter.
Thus, the rotor provides the major fraction of the lift of the hybrid helicopter in cruising flight, possibly also provides a small contribution to the propulsive or traction forces, and always operates with minimum drag.
By modifying the pitch of the blades of the propellers of the hybrid helicopter collectively and by the same amount, it is also possible to control the thrust generated by the propellers.
In contrast, the antitorque and yaw control functions are performed by using differential thrust exerted by the propellers, e.g. by the pilot operating a rudder bar.
Nevertheless, it is found that abusive use of the rudder bar can lead to a maneuver that subjects the helicopter to severe mechanical stress, or even to the hybrid helicopter yawing violently.
The technical field of the invention is thus the narrow technical field of flight controls for a hybrid helicopter.
Nevertheless, mention may be made of patents FR 2 476 013 and FR 1 132 452 that describe devices for allowing a control surface to move with large amplitude at low speed, while restricting said movement at high speed.
Thus, according to patent FR 2 476 013, an adjustment member limits the movement of a control means that might be moved by a pilot, with the limitation being a function of a signal that is derived from a dynamic pressure.
In contrast, patent FR 1 132 452 provides for limiting the effects of moving control means on a control surface as a function of the forward speed of the aircraft, such that identical movement of the control means gives rise to different movements of the control surface as a function of said forward speed.
Although they are of interest, those solutions would appear to be poorly adapted to the very particular context of a hybrid helicopter.
In addition, document FR 2 916 421 describes a control system having a member for generating a mean pitch setpoint.